Crankcase lubricants comprise basestock and additives that delay degradation of the basestock and improve its performance. Such additives typically include dispersant, overbased and neutral salts of organic acids, corrosion inhibitors, antiwear agents, antioxidants, friction modifiers, antifoamants, and demulsifiers. These additives may be combined in a package, sometimes referred to as a detergent inhibitor (or DI) package. The additives in such a package may include functionalized polymers, but these have relatively short chains, typically having a number average molecular weight Mn of not more than 7000.
Multigrade lubricants perform over wide temperature ranges. Typically, they are identified by two numbers such as 10W-30 or 5W-30. The first number in the multigrade designation is associated with a safe cranking temperature (e.g., -20.degree. C.) viscosity requirement for that multigrade oil as measured by a cold cranking simulator (CCS) under high shear rates (ASTM D5293). In general, lubricants that have low CCS viscosities allow the engine to crank more easily at lower temperatures and thus improve engine startability at those ambient temperatures.
The second number in the multigrade designation is associated with a lubricant's viscosity under normal operating temperatures and is measured in terms of the kinematic viscosity (kV) at 100.degree. C. (ASTM D445). The high temperature viscosity requirement brackets minimum and maximum kinematic viscosity at 100.degree. C. Viscosity at high temperatures is desirable to prevent engine wear that would result if the lubricant thinned out too much during engine operation. However the lubricant should not be too viscous because excessive viscosity may cause unnecessary viscous drag and work to pump the lubricant which in turn can increase fuel consumption. In general, the lower a lubricants' kV 100.degree. C., the better the scores that lubricant achieves in fuel economy tests.
Thus, in order to qualify for a given multigrade oil designation a particular multigrade oil must simultaneously meet both strict low and high temperature viscosity requirements that are set by SAE specifications such as SAE J300. The current viscosity limits set in SAE J300 are as follows:
______________________________________ SAE VISCOSITY GRADES SAE Maximum CCS kV 100.degree. C. kV 100.degree. C. viscosity Viscosity mm.sup.2 /s mm.sup.2 /s grade 10.sup.-3 Pa.s @ (.degree.C.) minimum maximum ______________________________________ 0W 3250 (-30) 3.8 -- 5W 3500 (-25) 3.8 -- 10W 3500 (-20) 4.1 -- 15W 3500 (-15) 5.6 -- 20W 4500 (-10) 5.6 -- 25W 6000 (-5) 9.3 -- 20 -- 5.6 &lt;9.3 30 -- 9.3 &lt;12.5 40 -- 12.5 &lt;16.3 50 -- 16.3 &lt;21.9 60 -- 21.9 &lt;26.1 ______________________________________
In the SAE J300 scheme multigrade oils meet the requirements of both low temperature and high temperature performance. For example, an SAE 5W-30 multigrade oil has viscosity characteristics that satisfy both the 5W and the 30 viscosity grade requirements--i.e., a maximum CCS viscosity of 3500.times.10.sup.-3 Pa.s at -25.degree. C., a minimum kV100.degree. C. of 9.3 mm.sup.2 /s and a maximum kV100.degree. C. of &lt;12.5 mm.sup.2 /s.
Presently, the viscosity characteristics of a lubricant are thought to depend primarily on the viscosity characteristics of the basestock and on the viscosity characteristics of the viscosity modifier. Of the other additives often found in lubricants only high molecular weight dispersants have been thought to influence viscometrics, and their influence has been deemed small in comparison to basestock and viscosity modifier.
The viscosity characteristic of a basestock on which a lubricating oil is based is typically expressed by the neutral number of the oil (e.g., S150N) with a higher neutral number being associated with a higher viscosity at a given temperature. This number is defined as the viscosity of the basestock at 40.degree. C. measured in Saybolt Universal Seconds. Blending basestocks is one way of modifying the viscosity properties of the resulting lubricating oil. For example a lubricant formulated entirely with S100N will have both a lower kV 100 and a lower CCS than a lubricant formulated entirely with a S150N basestock. A basestock comprised of a blend of S100N and S150N will have a CCS in between those of the straight cuts. The average basestock neutral number (ave. BSNN) of a blend of straight cuts may be determined according to the following formula: ##EQU1##
Merely blending basestocks of different viscosity characteristics may not enable the formulator to meet the low and high temperature viscosity requirements of some multigrade oils. The formulator's primary tool for achieving this goal is an additive conventionally referred to as a viscosity modifier or viscosity index (V.I.) improver. Usually, to reach the minimum high temperature viscosity required, it is necessary to add significant amounts of viscosity modifier which in turn results in increased low temperature viscosity. The ever increasing need to formulate crankcase lubricants that deliver improved performance in fuel economy tests is driving the industry to lubricants in the lower viscosity grades, that is 5W30, 5W20, and lower.
When the lubricant is a wide grade, e.g., 5W20 or 5W30, large amounts of viscosity modifier are nonetheless required. In these lubricants it is usual to reduce the basestock viscosity by blending in less viscous oils--i.e., to lower the average neutral number of the total basestock. If conventional mineral basestocks are used, it is usual to replace some or all of a higher viscosity basestock such as S150N basestock with a basestock of S100N or less to improve CCS performance in wide multigrades.
An alternative means of reducing the basestock viscosity and therefore improving CCS performance is to employ so-called non-conventional lubricants (or NCL). The American Petroleum Institute (API) in its Publication 1509 dated January 1993 entitled "Engine Oil Licensing and Certification System" (EOLCS) in Appendix E, 1.2 provided a classification of basestocks in a number of categories, which are widely used in the lubricant industry. Conventional mineral basestocks are in Groups 1 and 2; NCLs are basestocks that do not fall within those two Groups. Examples of NCLs are synthetic basestocks such as polyalphaolefin oligomers (PAO) and diesters and specially processed mineral basestocks such as basestocks that have been hydrocracked or hydroisomerised to give greater paraffinic content and lower aromatic content. These NCLs result in a smaller increase in volatility but are available only in limited quantities, are very expensive, and may not respond well to conventional antioxidant systems.
At the same time that fuel economy test performance is becoming more important, a need to reduce volatility of the lubricant has been identified. Oil volatility has been associated in the technical literature with both oil consumption and exhaust emissions both of which are undesirable. The two most significant factors influencing volatility are the solvent neutral number and viscosity index of the basestock. Basestocks with lower viscosity or low viscosity index are rich in more volatile components. One method used to measure volatility of a lubricant is the Noack method. Two standardized Noack methods are JPI Method 5S-41-93 and CEC Noak L-40-T-87. Those methods measure the percent mass lost in a sample after it has been held for a period of 60 minutes at 250.degree. C. with air being pulled through the sample. The official methods have good within lab repeatability and at this time poorer lab-to-lab reproducibility. Accordingly, for purposes of this invention all Noack measurements are to be made in instruments that have been calibrated with a reference fluid. The instrument should correlate within 6 wt. % to the weight loss observed for a reference fluid of known Noack volatility. One such reference fluid is reference lubricant RF 172 available from CEC, Brussels, Belgium and from Petrolab, Latham, N.Y. An oil is satisfactory if it yields a calibrated Noack volatility of not greater than 22 wt. % loss in an instrument that has been calibrated with fluids having standard volatilities.
Thus, the need for improved performance in fuel economy tests drives the blender to use basestock with lower solvent neutral numbers, while the need to reduce volatility drives the blender to use basestocks with higher solvent neutral numbers. The issue becomes particularly acute for lubricants in SAE J300 grades of 5W-30, 5W-20. To meet volatility requirements, lubricants of those grades are usually blended with basestocks having an average solvent neutral number of at least S105N.